1. Field of the Invention
The present invention relates to a high-strength cold-rolled steel sheet excellent in post-painting corrosion resistance.
2. Description of the Related Art
In recent years, a reduction in car body weight has increasingly been demanded, with the aim of improving collision safety and fuel consumption, in the automobile industry and a further enhanced strength has been demanded for a steel sheet that is one of the major component materials of a car body.
In particular, such a steel material is required to have, together with strength, press-formability that can cope with a complicated shape.
As prior arts that meet such requirements, for example, Japanese Patent No. 1177687, Japanese Unexamined Patent Publications No. S52-52115 and No. S52-69813 and others disclose a high-ductility high-tensile-strength steel sheet produced by complexly adding Si, Mn, etc.
Further, the phenomenon of generating strain-induced transformation of retained austenite and showing a large elongation in the event of: subjecting a low carbon steel to which Si and Mn are complexly added to an overaging treatment after two-phase zone annealing; changing a part of austenite to bainite; and finally forming a structure composed of ferrite, bainite and retained austenite, the so-called TRIP phenomenon, has been found recently. As technologies that make use of such a TRIP phenomenon, Japanese Patents No. 1925458 and No. 1430114, Japanese Unexamined Patent Publication No. H5-70556, and others, have been disclosed.
Furthermore, as the existence of elements such as C and Si, that are used in a high-strength steel sheet, deteriorate the surface quality of the steel sheet, as technologies to solve the problem, Japanese Examined Patent Publication No. H5-55570, Japanese Unexamined Patent Publication No. H10-280087 and others have been disclosed.